Paper is made on a Fourdrinier machine in which a slurry, or stock, comprising paper fibers, water, and optional additives for the paper is fed from a head box to a moving wire. The fibers are caught on the surface of the wire and become a paper web and the water drains through the wire and is discharged from the machine. When the paper web is dry enough to be self-supporting it is removed from the wire and carried through further processing stages in the machine and finally is dried and rolled up. One of the controlling factors governing the speed at which the machine is run is the length of time required to cause the paper to become sufficiently dry so as to be self-supporting. Paper is sold in a highly competitive market and the savings in manufacturing costs resulting from a small increase in machine speed can be a substantial competitive advantage. Accordingly, it is of great importance to speed up the removal of water from the paper web.
At the point where the stock is deposited on the wire from the head box, the wire is supported by a series of blades or foils which extend across the machine, with spaces between the blades to permit the water to pass therethrough. The wire subsequently passes across a suction box where additional water is removed by suction. The blades which are positioned underneath the moving wire close to the head box are generally of a type shown in Wrist U.S. Pat. No. 2,928,465. These blades are characterized by having a horizontal upper surface at the leading edge of the blade, this leading surface sometimes being called the land portion, followed by a trailing portion, sometimes called the foiling surface, which diverges from the horizontal an an angle up to about 5.degree.. It was found that this configuration for a foil caused water to be drawn through the wire more rapidly as a consequence of suction generated between the wire and the foiling portion. Such a blade removes water portion. the web or wire in two ways: The nose, or leading edge of the foil, bears against the wire and scrapes water from the lower surface of the wire; and the trailing portion causes a partial vacuum to pull water from the slurry side of paper fibers on the upper sides of the wire to the lower surface of the wire.
Since the first use of blades of the type shown by Wrist there have been many efforts to improve them. Foils have been produced with adjustable angles between the land and the trailing portion in an effort to improve performance. Because these blades are subject to wear they have been made easily removable and easily replaceable. Extra hard or wear-resistant materials have been employed in the manufacture of blades and wear-resistant coatings have been placed on the blade surfaces. Adjustable supports for such blades are shown, for example, in Dunlap U.S. Pat. No. 3,027,940. A wear-resistant coating or plate for the land portion of a blade is shown for example in Duncan U.S. Pat. No. 3,351,524. A wear-resistant insert fitted into a groove in the upper surface of the land and coplanar with the land surface is shown for example in Buchanan U.S. Pat. No. 3,446,702, and in Beacom U.S. Pat. No. 3,732,142. A wear-resistant insert in the trailing portion of the blade adjacent its intersection with the land is shown for example in Kienzl U.S. Pat. No. 3,738,911, and a wear-resistant tip is shown in Charbonneau U.S. Pat. No. 3,778,342. In addition, a wide variety of mounting means have been employed for quick change of blades. Thus, dovetails, as shown in Roecker U.S. Pat. Nos. 3,377,236, T-bars as shown in White 3,337,394, and other mounting means are employed.
In spite of all these changes in blade structure, there has not yet been much successful change in the contour of the upper surface of blades, and most or all blades heretofore in use have retained the contour of a horizontal land surface followed by a trailing foiling surface at a downward angle of 5.degree. or less. For example, the blade of Buchanan U.S. Pat. Nos. 3,446,702 employs the same contour as that of Wrist 2,928,465 with the apparent exception that the angle between the plane of the land portion and the foiling portion is accentuated, particularly as the blade is worn by use.